Systems and methods for loop length and bridged tap length determination of a transmission line

ABSTRACT

Through the use of a least squares minimization concept, the loop length, the number of bridged taps and length of the bridged taps on a transmission line can be determined from readily available modem data. In particular, the loop length, the number of bridge taps and the length of bridged taps can be estimated by comparing a measured frequency domain channel impulse response of the transmission line to a model of a loop that is comprised of multiple sections and multiple bridge taps.

This application is a continuation of application Ser. No. 09/755,172filed Jan. 8, 2001. This application claims the benefit of and priorityunder 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/174,866filed Jan. 7, 2000, entitled “Systems And Methods For Loop Length AndBridged Tap Length Determination Of A Transmission Line,” and U.S.Provisional Application No. 60/224,308 filed Aug. 10, 2000, entitled“Characterization Of Transmission Lines Using Broadband Signals In AMulti-Carrier DSL System.”

FIELD OF THE INVENTION

This invention relates to determination of transmission linecharacteristics. In particular, this invention relates to systems andmethods for determining loop lengths and bridged tap lengths of atransmission line.

BACKGROUND OF THE INVENTION

The collection and exchange of diagnostic and test information betweentransceivers in a telecommunications environment is an important part ofa telecommunications, such as an ADSL, deployment. In cases where thetransceiver connection is not performing as expected, for example, wherethe data rate is low, where there are many bit errors, or the like, itis necessary to collect diagnostic and test information from the remotetransceiver. This is performed by dispatching a technician to the remotesite, e.g., a truck roll, which is time consuming and expensive.

In DSL technology, communications over a local subscriber loop between acentral office and a subscriber premises is accomplished by modulatingthe data to be transmitted onto a multiplicity of discrete frequencycarriers which are summed together and then transmitted over thesubscriber loop. Individually, the carriers form discrete,non-overlapping communication subchannels of limited bandwidth.Collectively, the carriers form what is effectively a broadbandcommunications channel. At the receiver end, the carriers aredemodulated and the data recovered.

DSL systems experience disturbances from other data services on adjacentphone lines, such as, for example, ADSL, HDSL, ISDN, Ti, or the like.These disturbances may commence after the subject ADSL service isalready initiated and, since DSL for internet access is envisioned as analways-on service, the effect of these disturbances must be amelioratedby the subject ADSL transceiver.

SUMMARY OF THE INVENTION

Identifying, measuring and characterizing the condition of atransmission line is a key element of an ADSL deployment. In cases whenthe transceiver connection is not performing as expected, for example,the data rate is low, there are many bit errors, a data link is notpossible, or the like, it is important to be able to identify the looplength and the existence, location and length of any bridged tapswithout having to send a technician to the remote modem site to rundiagnostic tests.

This invention describes a system and method for estimating the looplength, the number of bridged taps and length of the bridged taps on atransmission line from readily available modem data. The loop length,the number of bridge taps and the length of the bridged taps can beestimated by comparing a measured frequency domain channel impulseresponse of the transmission line to a model of a transmission line thatis composed of multiple sections and multiple bridge taps. Thediagnostic and test information describing the condition of the line canthen be exchanged, for example, by two transceivers during a diagnosticlink mode, such as that described in U.S. patent application Ser. No.09/755,173 filed Jan. 8, 2001, entitled “Systems And Methods ForEstablishing A Diagnostic Transmission Mode And Communicating Over TheSame,” now U.S. Pat. No. 6,266,348, which is incorporated herein byreference in its entirety.

These and other features and advantages of this invention are describedin or are apparent from the following detailed description of theembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention will be described in detail, withreference to the following figures wherein:

FIG. 1 illustrates an exemplary multiple section loop with multiplebridged taps;

FIG. 2 illustrates a graph of the measured received reverb signal andthe theoretical model for downstream data;

FIG. 3 illustrates a graph of the measured received reverb signal andthe theoretical model for upstream data;

FIG. 4 is a functional block diagram illustrating an exemplary looplength and bridged tap length estimation system according to thisinvention;

FIG. 5 is a flowchart outlining an exemplary general method fordetermining loop length and bridged tap lengths according to thisinvention;

FIG. 6 is a flowchart outlining an exemplary method for estimating theloop length and bridged tap length in the upstream direction accordingto this invention; and

FIG. 7 is a flowchart outlining an exemplary method for estimating theloop length and bridged tap length in the downstream direction accordingto this invention.

DETAILED DESCRIPTION OF THE INVENTION

The exemplary embodiments of this invention will be described inrelation to the application of the invention to an ADSL transceiverenvironment. However, it should be appreciated that in general thesystems and methods of this invention will work equally well for anymultiple section loop with one or more bridged taps.

For example, during the ADSL modem initialization, the frequency domainchannel impulse response of the subscriber loop is measured at a set ofdiscrete frequency values. The measured frequency values are designatedas H_(m)(f_(i)), and f_(i)=i Δf, for i=0,1, . . . , k-1, where Δf is thefrequency spacing between adjacent samples.

FIG. 1 illustrates an exemplary model of a loop with N sections and Mbridged taps. The frequency domain model for the channel impulseresponse for the loop in FIG. 1 can be written as H(x,f), where f is thefrequency and the vector x contains the lengths (d_(i)) of the Nsections of the loop and the lengths (b_(i)) of the M bridged taps:x=[d₁, d₂ . . . ,d_(N), b₁, b₂ . . . ,b_(M),].Assuming that the number of sections of the multiple section subscriberloop, N, and the number of bridged taps, M, are known, an estimate ofthe optimal parameter vector x that best approximates the measuredchannel impulse response H_(m)(f_(i)) can be determined given the modelH(x,f). The optimal parameter vector set x* can be estimated byminimizing the norm of the difference between the measured and the modelfrequency response, at the discrete frequency values f_(i)=i Δf, fori=0,1, . . . , k-1. This minimization can be performed using theexpression:$x^{*} = {\min\limits_{x}{\sum\limits_{i = 0}^{k - 1}{{{{H_{m}\left( f_{i} \right)} - {H\left( {x,f_{i}} \right)}}}_{2}^{2}.}}}$If the number of the bridged taps on the loop is not known, by adoptinga large number of bridged taps in the model frequency response, andassuming that the minimization will converge to a solution with thecorrect number of bridged taps with non-zero length, the remainingbridge taps will have length zero.

The frequency domain model H(xf) can also incorporate the effect of, forexample, an imperfectly matched transmission line, by including theeffects of the load and source impedances.

More particularly, the loop characterization algorithms employ a modelbased approach to estimate the length of the loop and the lengths of upto two bridged taps. A channel characterization algorithm compares themeasured channel impulse response to the channel impulse response of aloop model consisting of a single-gauge wire and containing up to twobridged taps. However, it is to be appreciated that the basic model canbe extended to include multiple gauge wires and multiple bridged taps.The loop length and the bridged tap lengths are the parameters of thetheoretical channel impulse response. The system varies the parametersof the theoretical model and evaluates the difference between themeasured channel impulse response and the theoretical channel impulseresponse. The loop length/bridged tap lengths that minimize the errorfunction are then declared as the estimated values. The presence of abridged tap is declared if the bridged tap length is greater than apredetermined length, such as one hundred feet. This threshold forbridged tap detection was set experimentally. It was determined that formost loops there is a chance that a phantom bridged tap with a smalllength will be detected because of modeling inaccuracies and noise inthe measurement system. Since the lengths of these phantom bridged tapswere almost always below 100 ft, the exemplary threshold was set to 100ft. However, in general the threshold can be altered depending on theparticular operational environment and the complexity of the model.

There are two separate algorithms which perform loop characterizationfor downstream (DS) and upstream (US) data. For example, during modeminitialization, data collection software collects the reverb signal byaveraging K consecutive frames where K≧64. However, it is to beappreciated that as more averaging is performed, the less noisier themeasurement will be. However, since there is a prescribed number offrames in the standard modem training where the reverb signal istransmitted, the exemplary number of averages was set at 64. Thereceived reverb signal obtained in this way is an estimate of theimpulse response of the entire channel including the front-end responsesof the transmitting and receiving modems. The frequency domain receivedreverb signal is obtained in accordance with: $\begin{matrix}{{{Rx}(f)} = {\frac{1}{K}{\sum\limits_{k = 1}^{K}{{FFT}_{N}\left( {{rx}(n)} \right)}}}} & (1)\end{matrix}$where f is a dummy variable denoting frequency and rx(n), for n=1, . . ., N, are the samples of the time-domain received reverb signal within aframe, with N being the number of samples contained in a single frame.Equation 1 may contain a slight abuse of notation because in reality thefrequency variable f is not continuous but rather discrete and for thisreason the channel impulse response is available at a set of discretefrequencies, called tones, which are multiples of Δf=4312.5 Hz:f _(i) =i Δf, i=1, . . . ,N/2.  (2)The reverb signal is transmitted over a portion of the entire ADSLspectrum. For example, the reverb signal is available at 224 (96 inG.Lite) tones from f₃₂=32Δf to f₂₅₅=255Δf in the downstream channel andat 26 tones from f₆₌₆Δf to f₃₁=31Δf in the upstream channel. Thedownstream reverb signal is collected at the customer-premises equipment(CPE) and upstream reverb signal is collected at central office (CO).While there is no difference in the data collection process for theupstream or the downstream reverb signal, the characteristics of thesetwo data sets are quite different. Specifically, the downstream reverbdata contains significantly more information. Furthermore, there aremore samples of the frequency domain reverb signal available in thedownstream direction and these samples cover an extended range in thefrequency domain where the effects of bridged taps on impulse responsecan be easily detected. However, there is one crucial difference betweenthe upstream and the downstream data sets which complicates using thesame interpretation algorithm for both. In the downstream channel, thematching of the front-end impedance to the loop impedance tends to bebetter than in the upstream channel. This makes it possible to use asimplified channel model for the downstream channel. Unfortunately, theimpedance matching in the upstream channel is generally not as good asin the downstream channel and a more complicated channel impulseresponse should be used.

Due to these complications in channel modeling, and the lack ofsufficient data samples, the basic upstream channel characterizationalgorithm is limited in terms of estimation accuracy and the number ofbridged taps that can be detected. However, by extending the channelmodel to include multiple sections of varying gauges and/or more thantwo bridged taps, the presence of more than two bridged taps can bedetected and more accurate results for the lengths of individualsections of the loop determined if there is a change of wire gauge alongthe loop. The only trade off is that as the number of model parametersincrease, the computational effort needed to estimate the parameterswill increase as well.

The following describes the theoretical details leading to thederivation of the frequency domain channel impulse response of the modeland explains the channel characterization for both the downstream andthe upstream data in detail. Both the downstream and the upstreaminterpretation algorithms employ the same least squares minimizationconcept where the square of the error norm between the actual and thetheoretical channel impulse responses is minimized, but differ in thetheoretical channel impulse response used.

For the loop characterization for downstream data, an exemplary two-wireloop is characterized by its characteristic impedance:${Z_{0}(\omega)} = \sqrt{\frac{R + {{j\omega}\quad L}}{G + {{j\omega}\quad C}}}$And its propagation constant:γ(f)={square root}{fraction ((R+jωL)(G+jωC))}where ω=2πf is the radian frequency and R (resistance), L (inductance),G (admittance) and C (capacitance) are the frequency dependent constantsof the loop and vary with wire gauge. For a perfectly terminated loop,or a very long loop, with length d, and two bridged taps of lengths b₁and b₂, the transfer function of the loop H(d, b₁, b₂, f), is given by:$\begin{matrix}{{H\left( {d,b_{1},b_{2},f} \right)} = \frac{{\mathbb{e}}^{{- d}\quad{\gamma{(f)}}}}{\left\lbrack {2 + {\tan\quad{h\left( {b_{1}\gamma} \right)}}} \right\rbrack\left\lbrack {2 + {\tan\quad{h\left( {b_{2}\gamma} \right)}}} \right\rbrack}} & (3)\end{matrix}$

In logarithmic scale:log|H(d,b ₁ ,b ₂ ,f)|=log(2)−dγ(f)−log[2+tanh(b ₁γ)]−log[2+tanh(b₂γ)].  (4)

Note the linear dependence of the loop loss to the length of the cable.The actual transfer function of the loop can be measured during modeminitialization. Then the measured transfer function of the loop ismatched with that of a loop of length d with two bridged taps as givenin Eq. 3. In other words, determining d, b₁, and b₂ minimizes thefollowing least squares error criterion: $\begin{matrix}\left. {\min\limits_{d,b,b}\sum\limits_{i = {if}}^{il}} \middle| {{H\left( {d,b_{1},b_{2},f_{i}} \right)} - {{Rx}\left( f_{i} \right)}}|_{2}^{2} \right. & (5)\end{matrix}$where Rx(f_(i)) is the received reverb signal sampled at f₁=iΔf andi_(i) are i_(i) are the first and the last tones Rx(f_(i)).

An example of the operation of the algorithm for an exemplary loop isillustrated in FIG. 2. Displayed are the measured received reverb signalRx(f) and the theoretical model H (d, b₁,b₂,f) which were obtained byfinding the model parameters d,b₁,b₂ that best match the data.Specifically, the observed (dashed line) received reverb signal Rx(f) isplotted against the theoretical channel model (solid line) H(d,b₁,b₂,f)as functions of frequency for an exemplary 6000 ft loop with anexemplary single 1300 ft bridged tap. The exemplary loop consisted of a26 awg. 6000 ft wire with a 26 awg. 1300 ft bridged tap close to theCPE. The model parameters best matching the observed data were found tobe d=6000 ft, b₁=1300 ft and b₂₌₀ ft.

It follows from Eq. 5 that the interpretation algorithm basically does asearch over the variables d, b₁ and b₂ and finds the ones minimizing thecost function given below: $\begin{matrix}{{E\left( {d,b_{1},b_{2}} \right)} = {\sum\limits_{i = {if}}^{il}\left| {{H\left( {d,b_{1},b_{2},f_{i}} \right)} - {{Rx}\left( f_{i} \right)}}|_{2}^{2}. \right.}} & (6)\end{matrix}$Since the cost function E(d,b₁,b₂) is a nonlinear of d, b₁ and b₂, thefunction contains many local minima. Therefore, many well knownoptimization algorithms such as Gauss-Newton should not be used sincethese algorithms are unable to cope with multiple local minima and theyconverge to a local minimum of the cost function. In this exemplaryembodiment the global minimum of E(d,b₁,b₂) is desired. For this reason,a brute-force global minimization algorithm is used where the costfunction is sampled at the points (d^(p), b₁ ^(q), b₂ ^(r)), d^(p)=pΔD,b₁ ^(q)=qΔb₁ and b₂ ^(r)=rΔb₂ with p=1, . . . ,P, q=1, . . . ,Q and r=1,. . . ,R. Next the parameters (d^(p),b₁ ^(q),b₂ ^(r)) which result inthe minimum cost among the sampled values are chosen. This requiresevaluating the cost function at P×Q×R locations.

In order to be able to determine the theoretical transfer function ofthe loop, H(d^(p),b₁ ^(q),b₂ ^(r)), the frequency dependent propagationconstant γ(f) for a number of wires of different gauges needs to bestored. In an exemplary embodiment 24 awg. and 26 awg. wires are usedwhich require 4×N locations to store the real and the imaginary parts ofγ(f) for N ADSL tones. Additionally, the analog front end (AFE)compensation curves need be stored which occupy N locations in memory.Depending on where the algorithm is implemented, the loop transferfunction can be determined directly from Eq. 4, for example, if thealgorithm were implemented on a personal computer or workstation, or itmay be necessary to store the log[2+tanh(b₁γ)] terms in regularintervals as required by the sampling procedure for (d^(p),b₁ ^(q),b₂^(r)). For example, it is possible to pre-compute and store thelog[2+tanh(b_(i)γ)], i=1,2, from b₁=100 ft to b₁=2000 ft to in 100 ftintervals. Assuming low processor power, the log[2+tanh(b₁γ)] terms canbe predetermined and stored which take about 20×N locations for the realpart only. Therefore, in this exemplary embodiment, the total memory isabout (20+4+1+3)×N=28×N where 2×256 locations are needed to storeintermediate variables determined during the execution of the algorithm.

Although it will not be shown here, it is possible to simplify thecomputation of the cost function E(d,b₁,b₂,) so that only 12multiplications and 15 additions are needed. This means that the totalcomputational complexity of the algorithm is about P×Q×R×(11multiplications+15 additions) plus some additional start-up computationswhich are negligible compared to the above figure.

Unlike the downstream interpretation case, for upstream interpretationit is more accurate to assume that the line is not perfectly terminated.Specifically, the impedance mismatch at the transmitter-line connectionat the CPE modem and the impedance mismatch at the receiver-lineconnection at the CO modem become important factors that should be takeninto account. While the basic idea behind the channel characterizationalgorithm for the upstream data remains the same, and involves matchinga theoretical channel transfer function to the actual measured transferfunction, the computation of the theoretical channel transfer functionbecomes much more involved. As with the downstream interpretation case,the channel transfer function is again measured by averaging K frames ofthe received reverb signal as given by Eq. 1.

The theoretical model for the channel transfer function in the upstreamcase can be described in two steps. The first step consists of writingthe equations for the current and the voltage at the source (CPE),I_(s), V_(s), in terms of current and voltage at the load (CO), I_(L),V_(L), through the application of ABCD matrices: $\begin{matrix}{{\begin{bmatrix}V_{S} \\I_{S}\end{bmatrix} = {F^{s} \times A^{i} \times B \times A^{2} \times F^{L} \times \begin{bmatrix}V_{L} \\0\end{bmatrix}}},} & (7)\end{matrix}$where A^(i), B, F^(s) and F^(L) are 2×2 matrices whose elements arearrays of N elements. Here, A^(i) is a matrix representing the frequencydomain response of the ith section of the loop, B is the matrixrepresenting the response of the bridged tap and F^(S) and F^(L) are thematrices representing the frequency domain response of the analog frontend (AFE) hardware of the modem circuitry for TX (source) and RX (load)paths. From Eq. 7 the transfer function of the channel can be derivedand is given by: $\begin{matrix}{{{H\left( {d_{1},d_{2},b,f} \right)} = \frac{V_{L}}{V_{S}}},} & (8)\end{matrix}$where d₁ is the length of the section before a bridged tap and d₂ is thelength of the section after the bridged tap. Note that the COinterpretation algorithm uses a two-section, single bridged tap model.This is because of the limited number of frequency bins, fi=iΔf, fromtone i=6 to i=32, at which the transfer function is available.

Entries of the above matrices are given as follows:A ₁₁ ^(i) =A ₂₂ ^(i)=cosh(γd _(i))A ₁₂ ^(i) =Z ₀ sinh(γd _(i)), A ₂₁ ^(i) =A ₁₂ ^(i) Z ₀ ⁻²Entries of matrix B:B₁₁=B₂₂=1B ₁₂=0, B ₂₁ =Z _(j) ⁻¹(b)Where Z_(j) ⁻¹=tanh(bγ)/Z₀, and finally:F₁₁ ^(S)=F₂₂ ^(S)=1, F₁₂ ^(S)=0, F₂₁ ^(S)=Z_(S)F₁₁ ^(L)=F₂₂ ^(L)=F₁₂ ^(L)=0, F₂₁ ^(L)=Z_(L) ⁻¹The estimation algorithm minimizes the difference between the measuredand the actual transfer functions: $\begin{matrix}{\min\limits_{d,d,b}{{{{H\left( {d_{1},d_{2},b,f} \right)} - {{Rx}(f)}}}_{2}^{2}.}} & (9)\end{matrix}$

An example of the operation of the upstream loop length and bridged taplength estimation algorithm is illustrated in FIG. 3. Here the measuredreceived reverb signal Rx(f) and the theoretical model H(d,b₁,b₂,j),which was obtained by finding the model parameters d,b₁b₂ that bestmatch the data, are displayed. The exemplary loop consisted of 26 awg.7700 ft wire with a 26 awg. 600 ft bridged tap 5900 ft away from CO. Themodel parameters best matching the observed data were found to bed₁=7900 ft, d₂=0 ft and b=500 ft. Note that although the d₁ and d₂parameters found by the algorithm are different than their actualvalues, the actual values are d₁=5900 ft and d₂₌₁₈₀₀ ft, the sum ofd₁+d₂ is within 200 ft of the actual loop length. This exampleillustrates that even though the loop length is fairly accurate thelocation of the bridged tap is difficult to reliably estimate.

From the expressions leading to the theoretical channel transferfunction, H(d₁,d₂,b,f), it is clear that for the exemplary computationof the theoretical channel response Z_(S), Z_(L), Z₀ and γ, for 24 awg.and 26 awg., need be stored, and that Z_(j)(b₁) characterizing thebridged tap is dependent on the bridged tap length. Assuming anexemplary resolution of 100 ft in bridged tap length and a maximumexemplary detectable bridged tap length of 2000 ft, there are 20different Z_(j)(b₁) arrays. Finally, the sinh(.) and cosh(.) elements ofthe matrices A₁ and A₂ are stored. Then, assuming a 500 ft resolution inloop length and a maximum measurable loop length of 20,000 ft, thereshould be 80×46 locations for storing entries of A_(i). In total forstoring these variables there should be 108×46 memory locations,including storage for Rx(f) and H(d₁,d₂,b,f), and another 10×46locations are needed for storing intermediate variables during theexecution of the algorithm, giving a total of approximately 118×46memory locations for this exemplary embodiment.

FIG. 3 illustrates the observed (dashed line) received reverb signalRx(f) plotted against the theoretical channel model (solid line) H (d₁,d₂, j) as functions of frequency for an exemplary 7700 ft loop with asingle 600 ft bridged tap.

During the search process, P values for d₁, Q values for b and R valuesfor d₂ are selected and the cost function for each combination ofd₁,d₂,b determined. Thus, to determine the channel impulse responsethere are 4×(8×23 complex multiplications+4×26 complex additions).Therefore the total computational cost in this exemplary embodiment isP×Q×R×(32×26 complex multiplications+4×26 complex additions).

FIG. 4 illustrates an exemplary loop length and bridged tap lengthestimation system according to an embodiment of this invention fordownstream data. In particular, the loop length and bridged tap lengthestimation system 100 comprises a downstream loop length and bridged taplength determination device 200, an upstream loop length and bridged taplength determination device 300, a central office modem 20 and aconsumer-premises modem 30, connected by link 5, such as a twisted pair.The downstream loop length and bridged tap length determination device200 comprises a controller 210, an I/O interface 220, a storage device230, a reverb signal determination device 240, a loop length outputdevice 250 and a bridged tap output device 260, connected by link 5. Theupstream loop length and bridged tap length determination device 300comprises a controller 310, an I/O interface 320, a storage device 330,a reverb signal determination device 340, an impedance determinationdevice 350, a modem identification device 360, a loop length outputdevice 370 and a bridged tap output device 380, connected by link 5.

While the exemplary embodiment illustrated in FIG. 4 shows thecomponents of the loop length and the bridged tap length estimationsystem and associated components collocated, it is to be appreciatedthat the various components of the loop length and the bridged taplength estimation system 100 can be located at distant portions of adistributed network, such as a local area network, a wide area network,an intranet and/or the Internet, or within a dedicated loop length andbridged tap length estimation system. Thus, it should be appreciatedthat the components of the loop length and bridged tap length estimationsystem 100 can be combined into one device or collocated on a particularnode of a distributed network. As will be appreciated from the followingdescription, and for reasons of computational efficiency, the componentsof the loop length and the bridged tap length estimation system 100 canbe arranged at any location, such as in a general purpose computer orwithin a distributed network without affecting the operation of thesystem.

Furthermore, the links 5 can be a wired or a wireless link or any otherknown or later developed element(s) that is capable of supplyingelectronic data to and from the connected elements.

In operation, for determination of the loop length and the bridged taplength in the downstream direction, the controller 210, in cooperationwith the I/O interface 220 triggers initialization of the modem 20. Thereverb signal determination device 240, in cooperation with the modem20, the controller 210 and the I/O interface 220 determines a transferfunction by averaging K consecutive frames of a reverb signal. The looplength, a first bridged tap length and a second bridged tap length areinput from an input device (not shown) such as a computer, a laptop, aterminal, a transmission line testing device, or the like, or retrievedfrom the storage device 230.

The controller 210, in cooperation with the storage device 230, thendetermines the frequency domain propagation function for a specifiedwire gauge, and the frequency domain loop model. The calibrated andcompensated reverb signals in the frequency domain are stored in thestorage device 230 and the reference wire gauge input or retrieved fromthe storage device 230.

The controller 210, in cooperation with the storage device 230determines the number of elements in the Rx function and the differencebetween the actual and the measured transfer function. The loop lengthoutput device, in cooperation with the I/O interface then outputs theestimated loop length to, for example, a computer, a laptop, a terminal,a transmission line testing device, or the like. Additionally, thebridged tap output device outputs the estimated bridged tap length to,for example, a computer, a laptop, a terminal, a transmission linetesting device, or the like.

In operation, for determination of the loop length and bridged taplength in the upstream direction, the controller 310, in cooperationwith the I/O interface 320 triggers initialization of the modem 30. Thereverb signal determination device 340, in cooperation with the modem30, the controller 310 and the I/O interface 320 determines a transferfunction by averaging K consecutive frames of a reverb signal.

Next, the controller 310, in cooperation with the storage device 230,determines the frequency domain propagation function for a specifiedwire gauge, where the specified wire gauge is input or retrieved fromthe storage device 330.

The controller 310, in cooperation with the storage device 330 and theimpedance determination device 350, determines the frequency domainimpedance of the specified wire gauge. Then, the controller 310, incooperation with the storage device 330 and the impedance determinationdevice 350, determines the transmit impedance of the CPE modem and thereceive impedance of the CO modem.

The controller 310, in cooperation with the storage device 330,determines the matrix representing the frequency domain responses of thei^(th) section of the loop, the matrix representing the response of thebridged tap, and the F^(S) matrix representing the AFE circuitry for thesource (TX) and load (RX) paths and stores them in the storage device330, and estimates the transfer function H. The calibrated and thecompensated reverb signal in the frequency domain and the referencegauge of the wire are input or retrieved from the storage device 330.

The modem identification determining device 360 then determines theidentification of the CO modem collecting the upstream reverb signal,and the identification of the CPE modem transmitting the upstream reverbsignal. Knowing the number of elements in the Rx function, thecontroller 310 minimizes the difference between the actual and measuredtransfer functions, and outputs, with the cooperation of the loop lengthoutput device 370 and the bridged tap output device 380, the estimatedloop length and the estimated bridged tap length, respectively.

FIG. 5 illustrates an exemplary method of determining a loop length andbridged tap lengths. In particular, control begins in step S100 andcontinues to step S110. In step S110, the channel impulse response isestimated based on a measured reverb signal. Next, in step S120, thetheoretical channel impulse response of a loop model is determined usinga loop length and the bridged tap lengths. Then, in step S130, the looplength and the bridged tap lengths of the model are varied. Control thencontinues to step S140.

In step S140, the difference between the measured channel impulseresponse and the theoretical channel impulse is monitored. Next, in stepS150, the estimated values of the loop length and bridged tap length aredeclared based on the loop lengths and bridged tap lengths that minimizethe error function between the measured channel impulse response and thetheoretical channel impulse response. Control then continues to stepS160 where the control sequence ends.

FIG. 6 illustrates an exemplary method of determining the loop lengthand the bridged tap length for downstream data. In particular, controlbegins in step S200 and continues to step S210. In step S210, a modem isinitialized. Next, in step S220, a transfer function is determined byaveraging K consecutive frames of the reverb signal. Then, in step S230,the loop length is input. Control then continues to step S240.

In step S240, a first bridged tap length is input. Next, in step S250, asecond bridged tap length is input. Next, in step S260, the frequencydomain propagation function is determined for a specified wire gauge.Control then continues to step S270.

In step S270, the frequency domain loop model is determined. Next, instep S280, the calibrated and compensated reverb signals in thefrequency domain are input. Then, in step S290, the reference wire gaugeis input. Control then continues to step S300.

In step S300, the number of elements in the Rx function are input. Next,in step S310, the difference between the actual and the measuredtransfer function are determined. Then, in step S320, the estimated looplength is determined. Control then continues to step S330.

In step S330, the estimated bridged tap length is determined. Controlthen continues to step S340 where the control sequence ends.

FIG. 7 illustrates an exemplary method of determining the loop lengthand bridged tap length for upstream data. In particular, control beginsin step S500 and continues to step S510. In step S510, the modem isinitialized. Next in step S520, the transfer function is determined byaveraging K consecutive frames of the reverb signal. Then, in step S530,the frequency domain propagation function for the wire gauge in use isdetermined. Control then continues to step S540.

In step S540, the frequency domain impedance of the wire gauge isdetermined. Next, in step S550, the transmit impedance of the CPE modemis determined. Then, in step S560, the receive impedance of the CO modemis determined. Control then continues to step S570.

In step S570, the matrix representing the frequency domain responses ofthe i^(th) section of the loop are determined. Next, in step S580, thematrix representing the response of the bridged tap is determined. Then,in step S590, the F^(S) matrix representing the AFE circuitry for thesource (TX) and load (RX) paths are determined. Control then continuesto step S600.

In step S600, the transfer function H is estimated. Next, in step S610,the calibrated and the compensated reverb signal in the frequency domainare input. Then, in step S620, the reference gauge of the wire is input.Control then continues to step S630.

In step S630, the identification of the CO modem collecting the upstreamreverb signal is input. Next, in step S640, the identification of theCPE modem transmitting the upstream reverb is input. Then, in step S650,the number of elements in the Rx function are input. Control thencontinues to step S660.

In step S660, the difference between the actual and measured transferfunctions are minimized. Next, in step S670, the estimated loop lengthis determined. Then, in step S680, the estimated bridged tap length isdetermined. Control then continues to step S690 where the controlsequence ends.

As illustrated in FIG. 4, the loop length and bridged tap lengthestimation system can be implemented either on a single program generalpurpose computer, or a separate program general purpose computer.However, the loop length and bridged tap length estimation system canalso be implemented on a special purpose computer, a programmedmicroprocessor or microcontroller and peripheral integrated circuitelement, an ASIC or other integrated circuit, a digital signalprocessor, a hard wired electronic or logic circuit such as a discreteelement circuit, a programmable logic device such as a PLD, PLA, FPGA,PAL, a modem, or the like. In general, any device capable ofimplementing a finite state machine that is in turn capable ofimplementing the flowcharts illustrated in FIG. 5-7 can be used toimplement the loop length and bridged tap length estimation systemaccording to this invention.

Furthermore, the disclosed method may be readily implemented in softwareusing object or object-oriented software development environments thatprovide portable source code that can be used on a variety of computeror workstation hardware platforms. Alternatively, the disclosed looplength and bridged tap length estimation system may be implementedpartially or fully in hardware using standard logic circuits or VLSIdesign. Whether software or hardware is used to implement the systems inaccordance with this invention is dependent on the speed and/orefficiency requirements of the system, the particular function, and theparticular software or hardware systems or microprocessor ormicrocomputer systems being utilized. The loop length and bridged taplength estimation systems and methods illustrated herein, however, canbe readily implemented in hardware and/or software using any known orlater-developed systems or structures, devices and/or software by thoseof ordinary skill in the applicable art from the functional descriptionprovided herein and a general basic knowledge of the computer arts.

Moreover, the disclosed methods may be readily implemented as softwareexecuted on a programmed general purpose computer, a special purposecomputer, a microprocessor, or the like. In these instances, the methodsand systems of this invention can be implemented as a program embeddedon a personal computer such as a Java® or CGI script, as a resourceresiding on a server or graphics workstation, as a routine embedded in adedicated loop length and bridged tap length estimation system, a modem,a dedicated loop length and/or bridged tap length estimation system, orthe like. The loop length and bridged tap length estimation system canalso be implemented by physically incorporating the system and methodinto a software and/or hardware system, such as the hardware andsoftware systems of a dedicated loop length and bridged tap lengthestimation system or modem.

It is, therefore, apparent that there has been provided, in accordancewith the present invention, systems and methods for loop length andbridged tap length estimation. While this invention has been describedin conjunction with a number of embodiments thereof, it is evident thatmany alternatives, modifications and variations would be or are apparentto those of ordinary skill in the applicable arts. Accordingly, it isintended to embrace all such alternatives, modifications, equivalentsand variations that are within the spirit and scope of this invention

1. A loop length and bridged tap length estimation system comprising: acentral office (CO) modem and a customer-premises equipment (CPE) modemconfigured for multicarrier modulation communications therebetween; achannel impulse response device that determines a channel impulseresponse for a transmission line based on signals transmitted duringinitialization of the CO modem and CPE modem; a modeling device thatdetermines a theoretical channel impulse response; and a loop length andbridged tap determination device that determines a loop length and atleast one bridged tap length based on a comparison of the channelimpulse response and the theoretical channel impulse response.
 2. Thesystem of claim 1, wherein the comparison is based on an errorminimization function.
 3. The system of claim 2, where the errorminimization function is a least squares minimization.
 4. The system ofclaim 1, further comprising an impedance determination device thatdetermines at least one of a transmit impedance of the CPE modem and areceive impedance of the CO modem.
 5. The system of claim 1, furthercomprising a modem identification etermining device that determines atleast one of an identification of the CPE modem and an identification ofthe CO modem.
 6. The system of claim 1, wherein the theoretical channelimpulse response comprises variables representing a load and a sourceimpedance.
 7. The system of claim 1, where the channel impulse responseis estimated based on a measured reverb signal.
 8. The system of claim1, wherein the transmission line comprises at least one of at least onewire gauge and at least one bridged tap.
 9. The system of claim 1,wherein the transmission line is used for multicarrier modulationcommunications.
 10. A method for determining a loop length and at leastone bridged tap for a transmission line comprising: initializing acentral office (CO) modem and a customer-premises equipment (CPE) modemfor multicarrier modulation communications therebetween; estimating achannel impulse response based on signals transmitted during theinitialization step; determining a theoretical channel impulse response;comparing the theoretical channel impulse response and the estimatedchannel impulse response; and determining the loop length and at leastone bridged tap based on an error minimization function.
 11. The methodof claim 10, further comprising determining a transfer function of thetransmission line based on a reverb signal.
 12. The method of claim 11,wherein the transfer function is determined based on averaging apredetermined number of consecutive frames of the reverb signal.
 13. Themethod of claim 10, further comprising determining a frequency domainpropagation function for at least one wire gauge.
 14. The method ofclaim 10, further comprising determining at least one of a calibratedand a compensated reverb signal in the frequency domain.
 15. The methodof claim 10, further comprising determining the frequency domainimpedance of at least one wire gauge.
 16. The method of claim 10,further comprising determining a transmit impedance of the CPE modem.17. The method of claim 10, further comprising determining a receiveimpedance of the CO modem.
 18. The method of claim 10, furthercomprising one or more of: determining a matrix representing a frequencydomain response for a predetermined portion of the transmission line;determining a matrix representing a response of the at least one bridgedtap; determining a matrix representing an analog front-end circuit for asource and a load path; determining an identification of at least one ofthe CO modem and the CPE modem; and outputting an estimated loop lengthof the transmission line.
 19. The method of claim 10, further comprisingoutputting an estimated bridged tap length for the transmission line.20. A loop length and bridged tap estimation system comprising: meansfor initializing a central office (CO) modem and a customer-premisesequipment (CPE) modem for multicarrier modulation communicationstherebetween; means for estimating a channel impulse response based onsignals transmitted during the initialization step; means fordetermining a theoretical channel impulse response; means for comparingthe theoretical channel impulse response and the estimated channelimpulse response; and means for determining the loop length and at leastone bridged tap based on an error minimization function.